1. Field of the Invention
The present invention relates generally to window coverings and, more specifically, to an anti-reverse transmission apparatus for use in a lifting window blind (for example, a Venetian blind, roller blind, etc.) to let the user's biasing force be smoothly transmitted to the linking rod to lift or lower the blind body and, to prohibit reverse transmission of biasing force produced due to the gravity weight of the blind body.
2. Description of the Related Art
A conventional lifting window blind commonly uses a lift cord or string of beads for pulling by the user to control lifting or lowering of the blind body. Because the lift cord or string of beads is exposed to the outside of the blind body, an accident may happen when a child playing with the lift cord or tilt cord for fun.
Various window blinds with hidden cord members have been disclosed, and have appeared on the market. A window blind with hidden cord members, as shown in French Patent FR-2692002, is known comprising a transmission mechanism formed of an input shaft and an output shaft and installed in the headrail of the window blind. The input shaft can be directly rotated by the user (an operation rod may be directly fixedly or detachably connected to the input shaft for operation by the user to rotate the input shaft). The output shaft is driven by the input shaft to rotate a linking rod in the headrail, causing the linking rod to lift or lower the blind body.
In order to lock the blind body in position after each adjustment, a pull cord tension adjustment device may be used to offset the downward gravity weight of the blind body, preventing lowering of the blind body after position adjustment. However, the installation procedure of this structure of pull cord tension adjustment device is complicated. There is known a simple anti-reverse transmission apparatus using a worm and a worm gear to substitute for the aforesaid input shaft and output shaft. Theoretically, the engagement between a worm and a worm gear achieves anti-reverse transmission of a biasing force within a certain condition (the worm can rotate the worm gear with less force, however the worm gear requires a relatively bigger force to rotate the worm). In actual practice, excessively heavy of the component parts of the blind body or size limitation of component parts may affect the reverse transmission prohibition functioning of the transmission mechanism of the worm and worm gear, and the blind body may not be accurately stopped in position after adjusted to the desired elevation, or may slip down when touched by an external force accidentally.
Therefore, it is desirable to provide an anti-reverse transmission apparatus for window coverings that eliminates the aforesaid drawbacks.